1. Field of the Invention
The invention relates to a heating apparatus for heating and pressurizing a recording material bearing an unfixed toner image on the surface thereof.
More particularly, the invention relates to a toner image heating apparatus of a type which heats and fixes an unfixed toner image corresponding to desired image information formed and borne on the surface of a recording material (such as paper, printing paper, a transferring material sheet, an OHP sheet, glossy paper or glossy film) by a direct method or a transferring method by the use of a toner consisting of heat-fusable resin or the like by suitable image forming process means for electrophotography, electrostatic recording, magnetic recording or the like as a permanently fixed image on the surface of the recording material bearing the image, and an image forming apparatus such as a copying machine or a laser beam printer using the same.
2. Related Background Art
Laser beam printers and the like are known as an image forming apparatuses to which an electrophotographic process is applied. FIG. 11 of the accompanying drawings shows an example of the laser beam printers. In FIG. 11, the reference numeral 1 designates a photosensitive drum, the reference numeral 2 denotes an exposing apparatus, the reference numeral 3 designates a developing apparatus, the reference numeral 4 denotes a transferring member, the reference numeral 5 designates a cleaning apparatus, the reference numeral 6 denotes an electrifying member, the reference numeral 7 designates a fixing apparatus as a heating apparatus, the letter C denotes a sheet supplying cassette containing therein transferring materials which are recording materials to be supplied, the reference numeral 8 designates a sheet feeding apparatus, the letter P denotes a conveying path for the transferring materials, and the letter L designates a laser beam applied from the exposing apparatus 2.
In the above-described printer, the exposing apparatus 2 turns on/off and applies the laser beam L conforming to image information to the surface of the photosensitive drum 1 electrified to desired potential by the electrifying member 6, and eliminates charges to thereby form an electrostatic latent image on the photosensitive drum 1.
The developing apparatus 3 is comprised of a developer (hereinafter referred to as the toner), a developing container and a developing sleeve 31. The toner is supplied from the developing sleeve 31 in conformity with the electrostatic latent image, and a toner image is formed on the photosensitive drum 1. Thereafter, the toner image on the photosensitive drum 1 is transferred to the surface of the transferring material as the recording material by the transferring member 4. The unfixed toner image on this transferring material is heated and pressurized by the fixing apparatus 7, whereby it is permanently fixed on the surface of the transferring material, and the transferring material is discharged from the image forming apparatus.
On the other hand, any toner and paper dust or the like residual on the photosensitive drum 1 during the transfer are removed by the cleaning apparatus 5.
The fixing apparatus 7, as shown in FIG. 12 of the accompanying drawings, is provided with a fixing roller 10 and a pressure roller 20, and the fixing roller and the pressure roller are heated by heaters 13-1 and 13-2, respectively, provided therein. The temperature of the fixing roller 10 is detected as the surface temperature of the fixing roller 10 by a temperature detecting element 30-1 such as s thermistor brought into contact with the surface of the fixing roller, and the heater 13-1 is intermittently operated by a temperature control circuit 14-1 for the fixing heater, whereby the above-mentioned surface temperature is controlled to a predetermined temperature. On the other hand, the temperature of the pressure roller 20 is detected as the surface temperature of the pressure roller 20 by a temperature detecting element 30-2 such as a thermistor brought into contact with the surface of the pressure roller, and the heater 13-2 is intermittently operated by a temperature control circuit 14-2 for the pressure heater, whereby the above-mentioned surface temperature is controlled to a predetermined temperature.
In the above-described fixing apparatus, the transferring material T bearing the toner image t thereon is directed to a contact nip portion (fixing nip) N between the fixing roller 10 and the pressure roller 20, and is heated and pressurized by this nip portion, whereby the toner image is fixed on the surface of the transferring material.
When in the laser beam printer provided with such a fixing apparatus, a higher printing speed is contrived to thereby secure a fixing property, electric power is preferentially supplied to the fixing heater (the heater in the fixing roller) side so that the surface temperature of the fixing roller of the fixing apparatus can be maintained constant. Consequently, electric power supplied to the pressure heater (the heater in the pressure roller) side is limited and therefore, it becomes impossible to maintain the surface temperature of the pressure roller at a sufficiently high temperature.
On the other hand, the amount of heat necessary for fixing differs depending on the basis weight of the transferring material, particularly paper, and therefore, a great amount of heat is necessary for paper having a great basis weight of e.g. 128 g/m2 or 199 g/m2 or the like (hereinafter referred to as the thick paper). Also, when paper having unevenness on the surface thereof for the purpose of a sense of high quality or an improved sense of quality (hereinafter referred to as the rough paper) is to be printed, it is difficult for heat to be transferred to the paper and therefore, the secure a sufficient fixing property, it is necessary to set the temperature of the fixing roller or the pressure roller at a high level. Thus, to sufficiently fix an image on the thick paper or the rough paper, the surface temperature of the pressure roller is important and therefore, heretofore a thick paper mode, a rough paper mode or the like has been provided and has been arbitrarily settable by a user. Such a special mode sets the controlled temperature of the fixing roller at a high level or lowers the throughput to thereby set the temperature of the pressure roller at a high level.
However, when copying with the thick paper or the rough paper by a particular mode (particular controlled temperature and throughput), the surface temperature of the pressure roller is governed by differences in the heat capacity and heat transfer efficiency of the paper. That is, not only the fixing property does not become constant between different kinds of paper such as the thick paper and the rough paper, but also unevenness occurs to the curl amount of the paper. Also, if the particular mode is set for paper having a great basis weight or paper of bad heat transfer efficiency, the entire throughput becomes slow, or conversely, if the throughput is set fast to a certain extent in accordance with paper having a relatively small basis weight or paper of relatively good heat transfer efficiency, there will arise the problem that paper like the former cannot be coped with.